Biological pest control

Ant and tech

How weaver ants could come to the rescue of African mango farmers

THE humble but industrious ant has long served as a metaphor for the economic virtues of simplicity, parsimony and diligence. But in the case of weaver ants in Africa, this description may be more than just a metaphor. According to a study published in the latest issue of the Journal of Economic Entomology, by Paul van Mele of the Consultative Group on International Agricultural Research and his colleagues, African mango farmers could increase their harvests by as much as two-thirds with the help of these doughty insects.

Mangoes in Africa, as elsewhere, often fall prey to fruit flies, which destroy about 40% of the continent's crop. In fact, fruit flies are so common in African mangoes that America has banned their import altogether, to protect its own orchards. African farmers, meanwhile, have few practical means to defend their fruit. Chemical pesticides are expensive. And even for those who can afford them they are not that effective since, by the time a farmer spots an infestation, it is too late to spray. Added to that, spraying tall trees is a much more complicated and unhealthy business than treating low-growing fruit and vegetables.

Agricultural scientists have also looked at controlling fruit flies with parasitic wasps. But the most common ones kill off only about one fly in 20, leaving plenty of survivors to go on the rampage. Lethal traps baited with fly-attracting pheromones are another option. But they, too, are expensive. Moreover, all these methods require farmers to detect the presence of fruit flies, and to identify them as the main threat to their crop—no mean feat when most of the action is taking place in dense, leafy canopies ten metres off the ground. Instead, most farmers simply harvest their fruit early, when it is not yet fully ripe. This makes it less vulnerable to the flies, but also less valuable.

Farmers whose trees are teeming with weaver ants, however, do not need to bother with any of this. In a survey of several orchards in Benin, Dr van Mele and his colleagues found an average of less than one fruit-fly pupa in each batch of 30 mangoes from trees where weaver ants were abundant, but an average of 77 pupae in batches from trees without weaver ants. The weaver ants, it turns out, are very thorough about hunting down and eating fruit flies, as well as a host of other pests. The only drawback is the ants' painful bite, which can be avoided by harvesting fruit with poles, rather than climbing trees.

Weaver ants have been used for pest control in China and other Asian countries for centuries. The practice has also been adopted in Australia. But Dr van Mele argues that it is particularly suited to Africa since weaver ants are endemic to the mango-growing regions of the continent, and little training or capital is needed to put them to work. All you need do is locate a suitable nest and run string from it to the trees you wish to protect. The ants will then quickly find their way to the target. Teaching a group of farmers in Burkina Faso to use weaver ants in this way took just a day, according to Dr van Mele. Those farmers no longer use pesticides to control fruit flies, and so are able to market their mangoes as organic to eager European consumers, vastly increasing their income. The ants, so to speak, are on the march.